Core wound paper products are in frequent use in today's society. Such products often have a hollow tubular core about which a roll of the product to be used is wound. Particularly popular core wound paper products include toilet tissue and paper towels.
Core wound paper products are frequently made of two plies, which are typically identical, and are superimposed in face-to-face relation to form a unitary laminate. Dual plies having a particular aggregate thickness are generally preferred over a single ply having the same thickness, because the resulting dual ply laminate is softer than the single ply product. Of course, comparable absorbency and tensile strength can be obtained, whether the total thickness is comprised of one ply having a predetermined thickness or of two plies, each having approximately one-half the predetermined thickness.
The superimposed plies may be adhesively joined in face-to-face relation to prevent each from separating from the other ply. However, adhesive joining increases both the manufacturing cost and the stiffness of the core wound paper product. Therefore, it is desirable that the core wound paper product have superimposed plies which remain joined in face-to-face relationship without the expense and stiffness of adhesive, and yet present a unitary laminate to the consumer during use.
An additional manufacturing consideration is that the consumer usually does not wish to use the entire roll of paper product at once. To aid the consumer in selecting and dispensing the proper portions of the product, the roll of paper product is provided with lines of weakness generally parallel the axis of the core about which the paper product is wound. The lines of weakness may comprise perforations which divide the core wound paper product into individual sheets which are joined across the perforations, yet are easily separated from the adjacent sheet.
The perforations provide for incremental dispensing of individual and multiple sheets of the product. This feature allows the consumer to conveniently dispense a particular quantity of the product at his or her convenience.
The perforations may be made by perforator blades employed during the manufacturing process. The perforator blades are typically mounted on a rotating cylinder and have alternately spaced teeth and notches across the total width of the perforator blade. The teeth of the perforator blade are responsible for the small cuts which define and divide adjacent sheets of the paper product, while the notches of the perforator blade are responsible for the lands of the paper product which bridge adjacent sheets and hold the roll of sheets together.
During the perforating step of the manufacturing process, the paper product is interposed between the perforator blade and a rigid anvil. The rotating perforator blades strike the paper product while it is held against the anvil, and cut through the thickness of the paper product, at the teeth of the perforator blade. The transverse lines of weakness dividing and defining adjacent sheets are formed when the teeth of the perforator blade cut through the paper product to form the perforations.
The perforator blade, and the associated manufacturing process, control certain properties of the finished product. It is important that the perforator blades produce desirable properties in the finished product--so that consumer acceptance of an otherwise suitable product is not diminished by, for example, poor dispensing caused by the type or nature of the perforations imparted by the perforator blades. Furthermore, it is important that the perforator blades be as long lasting as reasonably achievable, so that unduly frequent changeout of the perforator blades, downtime of machinery, or other maintenance is not required.
The relative size, including the length, width and thickness of the perforator blade teeth and notches control several properties directly related to the dispensability and performance of the paper product. For example, if the notches of the perforator blade are too narrow, for a given total notch width, the tensile strength between adjacent sheets of the paper product will be too great, and it will be difficult for the consumer to tear one sheet of the paper product from the remainder of the roll of the product. Also, if the tensile strength of the perforations joining the adjacent sheets is too great, the sheet may not tear along the transverse line of weakness as desired, but rather may tear through the middle of the sheet, resulting in an undesired ragged appearance and two sheets of nonuniform size.
Additionally, if the notch width is too great, for a given total notch width, the bond strength joining superimposed plies in face-to-face relation across the cuts of the perforations may be too small and the superimposed plies of the sheet may easily separate. If the two plies separate, an individual ply is typically insufficient for the consumer's desired end use.
Thus, there is a tension between two desired properties of the core wound paper product. If the notch width is too narrow, the tensile strength between adjacent sheets becomes too great, while the bond strength between superimposed plies is improved. Clearly, a need exists to find a perforator blade which can accommodate the properties of both of these diametrically opposed needs.
Furthermore, it is desired to reduce the amount of lint created during manufacturing. The lint can create a hygiene problem if the amount of lint becomes excessive. The hygiene problem stems from the accumulation of lint which can lead to spontaneous combustion or fall, in clusters, from higher elevations and be incorporated into the paper product in a clump.
To reduce the amount of lint created during manufacturing, cellulosic (and any other) fibers of the paper product need to be securely held to the sheet during and after the perforating step. To securely hold the fibers, it is desirable to utilize relatively narrow teeth in the perforator blade. This introduces yet another parameter that must be taken into consideration when selecting the proper geometry for the perforator blade.
Yet another parameter controlled by the perforator blade is the visual appearance of the free edge of the sheet remaining after an adjacent sheet is removed by tearing through the perforations. The consumer desires an aesthetically pleasing free edge in the product after dispensing. A more aesthetically pleasing free edge typically requires a smoother, less jagged appearance between the cut and uncut areas at the edge of the sheet.
Several attempts have been made in the art to allow a wide selection in the parameters determining the geometry of the perforator blade used in the perforating process. For example, one supplier of perforator blades, the Kinetic Company of Greendale, Wis., has at least six different parameters available for selection (within reasonable limits) by the end user of the perforator blade. As illustrated in the advertising literature, an end user ordering a perforator blade from the Kinetic Company can select: the total number of notches per side of the perforator blade, the width of each notch, the thickness of the perforator blade perpendicular to its width, the overall height of the perforator blade in the other direction perpendicular to its width, the total width of the perforator blade, and whether or not the distal edges of the teeth of the perforator blade are straight (as illustrated in the accompanying figures) or are concave arcuate with a radius to be selected.
One attempt, illustrated in U.S. Pat. No. 4,963,406 issued Oct. 16, 1990 to Gooding, Jr. et al. is directed to perforated paper products having three parallel lines of perforations. A sheet of the product is torn from the adjacent sheet by tearing along the central line of the three, so that the other two lines of perforations maintain the bond between the plies. The perforations have a width of 1.5 millimeters to 2.5 millimeters (0.06 to 0.1 inches) on a spacing of 0.8 millimeters to 1.3 millimeters (0.03 to 0.05 inches) for toilet tissue, and a width of 0.3 millimeters to 0.4 millimeters (0.01 to 0.15 inches) on a spacing of 0.8 millimeters to 1.3 millimeters (0.03 to 0.06 inches).
However, this teaching triples the complexity of the perforating process. Three perforating blades are required, in the place of each single perforating blade used in the prior art. More frequent perforating blade breakage, and consequently, machine downtime to replace broken perforating blades will occur with a triple perforating blade apparatus, as taught in this reference.
Another teaching can be found in single ply, continuous feed, Z-fold computer paper sold by Willamette, Industries, Inc. of Willamette, Ill. This paper has perforations dividing adjacent sheets and the sprocket feed strips. The cuts of the perforations are about 0.17 millimeters (0.0065 inches) in width and the lands are about 0.17 millimeters (0.0065 inches) in width, and the paper has about 340 lands per 113 millimeters (4.46 inches) of paper width. However, due to the relatively low total land width across the entire width of the paper, the perforation tensile strength of this paper is too low for core wound paper products, such as toilet tissue and paper towels.
Several recent and specific attempts to optimize the perforator blade geometry can be observed in the art. For example, 114 millimeter (4.5 inches) wide Kleenex brand toilet tissue made by the Kimberly-Clark Corporation of Neenah, Wis., are made utilizing a perforator blade having a tooth width of about 1.0 millimeters (0.04 inches) and a notch width of about 0.6 millimeters (0.03 inches) and a total notch width of about 47.6 millimeters (1.88 inches). However, utilizing a perforator blade according to the present invention, the perforation tensile strength joining adjacent sheets is maintained, and significant improvements in the perforation bond strength between superimposed plies may be obtained over the prior art--while improving the overall perforator blade life.
Accordingly, it is an object of this invention to provide a perforator blade which optimizes both diametrically opposed properties of perforation tensile strength between adjacent sheets and perforation bond strength between superimposed plies. It is also an object of this invention to provide a perforator blade which has a life at least as long as those of perforator blades according to the prior art.
Finally, it is an object of this invention to provide a perforator blade which diminishes the hygiene problems that occur during manufacturing and are caused by the lint produced during the perforating process. Yet the perforator blade should yield a perforation, which when visible to the consumer, has a more aesthetically pleasing appearance than perforations made by perforator blades according to the prior art.